1) Field of the Invention
This invention relates generally to structures and method of making circuit fabrication masks and more particularly some embodiments relate to the structures and methods of making of phase shifting circuit fabrication masks.
2) Description of the Prior Art
Improvements in photolithography have increased the density and enhanced the performance of semiconductor devices by shrinking integrated circuits (ICs). As described by the Rayleigh criterion, the minimum critical dimension (CD) which can be resolved by a wafer stepper is directly proportional to the wavelength of the illumination source and inversely proportional to the numerical aperture (NA) of the projection lens. However, diffraction tends to degrade the aerial image when the CD becomes smaller than the actinic wavelength. The actinic wavelength is the wavelength of light at which a mask is used in a wafer stepper to selectively expose photoresist coated on a substrate, such as a Silicon wafer. As needed, a resolution enhancement technique (RET), such as a phase-shifting mask (PSM), may be used to achieve a wider process latitude. Unlike a binary mask that only uses Chrome to control the amplitude of light transmitted through a quartz substrate, a PSM further modulates the phase of light to take advantage of destructive interference to compensate for the effects of diffraction.
An alternating PSM (AltPSM) is a type of PSM that is particularly helpful in improving contrast when patterning very small CDs, such as the gate length of a transistor in a device. AltPSM introduces a phase shift of 180 degrees between the light transmitted through adjacent clear openings so destructive interference can force the amplitude between the two images to zero. A phase shift of 180 degrees is implemented by creating a difference in the optical path lengths through adjacent openings in an opaque layer, such as Chrome. A subtractive process may be used to etch a trench into the quartz substrate in alternate openings. However, incident light may scatter off the sidewalls and bottom corners of the etched trench and cause an imbalance in the aerial image that varies as a function of focus. Such a waveguide effect may be manifested as a CD error and a placement error.
The intensity and phase in the aerial image of an AltPSM may be balanced in various ways. A selective biasing approach enlarges the CD of the etched opening relative to the unetched opening to balance the aerial image. An etchback approach undercuts the edges of the chrome in both openings to balance the aerial image. A dual-trench approach etches a deep trench in the phase-shifted opening and a shallow trench in the non-phase-shifted opening to balance the aerial image.
The basic concept of increasing the resolution of a lithographic image is to modify the optical phase of the mask transmission. In an alternating PSM, alternating areas of chrome and 180 degree-shifted quartz are employed to form features on the wafer. Contrast is increased because the light diffracted into the nominally dark area will interfere destructively with the light diffracted from the clear area. The AltPSM is the “strongest” PSM technology and can improve the resolution of a given wafer exposure system by approximately 40%.
The problem of aerial image intensity imbalance through focus with AltPSM has been well-documented. One known solution for the intensity imbalance issue is to provide undercuts beneath the chrome and to bias (i.e., to thin) the trench chrome opening. However, undercuts and biasing limit the minimum chrome size and hence contributes to the chrome peeling issue.
FIG. 9 shows some examples of the methods used to correct the image imbalance. The graph on the left shows the intensity imbalance for the PSM (shows below on the left.). The graph on the right shows improved image balance. Below are 3 examples of methods to improve; 1) undercut, 2) bias and 3) undercut an bias.
The apparently more relevant technical developments in the patent literature can be gleaned by considering the following patents.
U.S. Pat. No. 6,531,250: Kim and U.S. 20010009745A1: Kim—Half tone phase shift mask having a stepped aperture-Half-tone phase shift mask used in forming predetermined pattern of semiconductor integrated circuits, includes light-transmitting phase shift pattern defining stepped aperture.
U.S. Pat. No. 5,514,500: Ham—Half-tone type phase shift mask and method for fabricating the same—Mfr. of half-tone type phase shift masks—by forming phase shift layer on transparent substrate, forming light screen on phase shift layer, selectively etching light screen and phase shift layer to form pattern. The chrome pattern has a step.
U.S. Pat. No. 5,281,500: —Cathey, David A.; —Method of preventing null formation in phase shifted photomasks—Preventing zero formation in phase-displacement photoresist masks—by proving a transparent substrate with dark features in relief, and delimiting at least one end of upwardly-projecting clear phase displacement features, etc.
U.S. Pat. No. 5,487,962: —Rolfson, J. Brett; —Method of chromeless phase shift mask fabrication suitable for auto-cad layout—: Chrome-less phase-shift masks suitable for auto CAD layout—comprising raised shifters formed with vertical edge and tapered edge.
U.S. 20040073884A1: —Kroyan—Phase shifting mask topography effect correction based on near-field image properties—Image intensity imbalance correction method for phase shifting mask in e.g. deep UV lithography, involves computing near-field image for pair of shifters, based on which bias is determined for phase shifters. —The patent shows undercuts for an alt-psm.
U.S. Pat. No. 6,458,495: —Tsai, Wilman;—Transmission and phase balance for phase-shifting mask—Phase-shifting mask with balanced transmission and phase, has trenches with vertical sidewall profiles which are retrograde
U.S. Pat. No. 6,627,359: —Kokubo, Haruo—Phase-shift photomask manufacturing method and phase-shift photomask—Phase-shift photomask manufacture for forming resist pattern on wafer, involves wet-etching light transmission section on substrate to set depth of recesses formed on section to satisfy specific relationship.
U.S. Pat. No. 6,410,191: —Nistler, John L.—Phase-shift photomask for patterning high density features. —Phase-shift photomask for patterning high density features and manufacture. The phase shifting region includes sloped sidewalls having a slope of less than about 85°.